I'm looking to add a little extra reach to my 200mm Canon prime, to do the occasional tighter crop on action/ hobbyist wildlife, etc. I'm considering the Canon 2x vs the Sigma 2x.Does anybody have any experience with this combination, I would Iove to read your thoughts. Currently shooting on an original 5D. Thanks in advance.

You can add a 1.4x Extender or TC with very little reduction in IQ, but the 2X variety often doesn't work as well as you might hope. The new Canon MK III extenders are the best ever (haven't tried Sigma), but they're probably most suitable for supertele lenses. I suggest you pick up a Kenko Teleplus Pro 300 DG (or DGX) 1.4x TC or used Canon 1.4x Extender Mk II for starters (see Monito's post, below). You can add a second 1.4x TC to get to 2x and retain the extra flexibility of having both 1.4x and 2x available. Many people (myself included) have reported better results with stacking 1.4x TC's than with a 2x TC.

Re recommendation for either original or Mark II 1.4x: everything I've read indicates that optically the Mark II is much better than the original, more so than the Mark III improves over the Mark II (which it does). So I would not recommend buying a used original; instead go for the Mark II.

I've only used a Canon 1.4x Mark II (on a different lens) so I can't speak from specific experience, but on the lens I use it on it is very effective and has little effect on image quality.

I just checked the EF Museum and the 1.4x II does have different optics than the Mk I. In the "block diagrams", you can see the Mk II has a curved front element, while the Mk I looks flat. Also, the Mk II description is a dead giveaway:

So, back to the original recommendation; if you decide to get a 1.4x to start, then Alan's (Monito) recommendation of a EF 1.4x Extender Mk II is probably the best start. If you ever want to add a second 1.4x TC, then you'll have to pick up a "generic", as you can't stack Canon 1.4x Extenders.

IMO, the Kenko DG/DGX is the best "generic". This view is shared with many other posters in past FM threads. IQ with the Kenko DG 1.4x is virtually idential to the Canon 1.4x Mk I. I'm pretty sure that I posted some comparisons here, a few years ago. I now have Mk III versions of both Canon Extenders, and still use the Kenko DG. AFAIK, the DG and contemporary DGX are the same, except for new coatings on the 'X'. The DG is noticeably better than the Tamron SP AF 1.4x (7-element) TC.

I have a couple examples of the 200 f2.8 II paired with the Canon 2x II. I have since purchased the 2x III which does look a little better but haven't taken it out with the 200 yet, only with my 300 f2.8.

The results are reasonably good for many applications if you really need the reach. Good light helps of course. The Canon 200 f2.8 II is a pretty nice little lens for the size and price. Easy to carry in a small backpack. Throw in the two converters and you have 200, 280 and 400 in a pretty small package.

Answering my own question...
here's a couple of grabs from the recent Classic Racing Day @ Dragon Track, Kaloianovo, Bulgaria.
5D, 200/2.8 II, Canon 2x II
Amazing what you can do to a RAW image with the little help of the contrast slider at post

Using a 5D III I have had excellent autofocus and very good image quality using my 135 L and the 1.4 mark 2 to shoot soccer and lacrosse in good light. Best at f/5.6, and a bit softer if wider, but still quite good at f/4.0. But with only 189mm I need to let the action come to me.